Placing material at a location below ground may be done in order to enhance flow, notably in the context of hydraulic fracturing, or may be done in order to restrict flow in the context of various operations including plugging, diversion, control of lost circulation and zonal isolation.
Hydraulic fracturing is a well established technique for reservoir stimulation. In normal practice a solid proppant is mixed with the fracturing fluid at the surface and pumped under conditions of very high shear. The proppant-laden fluid then flows down the wellbore under conditions of lower shear. Subsequently it turns and flows out of the wellbore and into the fracture in the formation. Entry to the fracture may be associated with an increase in shear, in particular if the wellbore is cased and the fluid passes through perforations in the wellbore casing to enter the fracture. Once the fluid enters the fracture the fluid is subjected to much less shear. The proppant carried by the fluid becomes packed in the fracture. Subsequently pumping is discontinued, allowing the fracture to close onto the proppant packed in the fracture. Outflow of fluid from the reservoir is allowed to begin, or is restarted, displacing the aqueous fracturing fluid as it does so. The porous pack of proppant in the fracture provides a flow path to the wellbore and so the fracture enhances flow from the reservoir.
It is known to apply a surface treatment to some of the proppant so that the particles of proppant in the pack adhere to one another. This is done in order to minimise the return of proppant particles, especially fines, as liquid flows out of the fracture (“proppant flowback control”). For example U.S. Pat. No. 6,725,931 teaches that a hardenable resin should be applied to all the proppant and should remain tacky after hardening in order to trap any fines passing through the proppant pack. U.S. Pat. No. 7,392,847 teaches an alternative form of surface modification of proppant particles, but again with the objective that the proppant particles in the fracture adhere together.
Whether or not the proppant particles are caused to adhere to one another, the proppant pack needs to be porous to allow outflow of fluid through the proppant pack in the fracture. It is normal practice to employ solid material of controlled particle size distribution in order that the proppant pack has adequate fluid conductivity, i.e. is adequately porous.
It has been appreciated for many years that conductivity through a propped fracture could be enhanced if some way could be found to cluster the proppant at localised areas within the fracture, leaving open pathways or channels for flow between the clusters of proppant (which are sometimes termed “pillars” of proppant). This has come to be referred to as heterogeneous proppant placement. An early document which proposed this is U.S. Pat. No. 3,850,247 published in 1974 which proposed achieving heterogeneous proppant placement by injecting a carrier liquid with proppant particles suspended therein, alternately with a displacement liquid which does not include suspended proppant.
U.S. Pat. No. 6,776,235 teaches sequentially injecting into the wellbore alternate stages of fracturing fluids having a contrast in their ability to transport proppant or having a contrast in the amount of transported proppant. U.S. Pat. No. 7,213,651 also teaches injection of two fracturing fluids alternately.
US published application 2008/0135242 teaches the introduction of a proppant and also a proppant-spacing filler termed a “channelant” which is subsequently removed to leave open channels between islands of proppant.
One circumstance in which it is desired to block a flow path below ground is in addressing the problem of lost circulation. As is well known, when a well is being drilled, the drilling mud is circulated down the drillstring, through the drill bit and back up the surrounding annulus. If the drill penetrates a very porous formation or a formation with cavities, some of the expensive drilling fluid may be lost. There are various approaches to this fluid loss problem and some examples have been disclosed in U.S. Pat. Nos. 5,439,057, 5,680,900, 6,814,145 and 6,176,537.
It is sometimes desired to block a flow path within a reservoir in order to divert flow elsewhere, to block water entry or to isolate one zone from another. Some examples of approaches to plugging and sealing flow are disclosed in U.S. Pat. Nos. 4,600,057, 5,358,051, 7,331,390 and 7,287,586.
In some circumstances it is desired to block the path of flow within a wellbore. Notably this is done when making multiple fractures of a single well, for example at different depths or at intervals along a horizontal well. A temporary plug can be created with a tool, although the cost of inserting and later removing a tool may be considerable so that placing material in the wellbore to plug may be a more economical option. There are a variety of techniques in this category and some examples have been described in U.S. Pat. Nos. 7,380,600 and 7,273,104.